Laura Gagliardi*, and Pekka Pyykko1

Transcription

1 4690 J. Phys. Chem. A 2002, 106, η 5 -N 5 - -Metl-η 7 -N 7 3- : A New Clss of Compounds Lur Gglirdi*, nd Pekk Pyykko1 Diprtimento di Chimic G. Cimicin, UniVersità di Bologn, Vile F. Selmi 2, I Bologn, Itly, nd Deprtment of Chemistry, UniVersity of Helsinki, P.O.B. 55 (A. I. Virtsen ukio 1), FIN Helsinki, Finlnd ReceiVed: NoVember 12, 2001; In Finl Form: Februry 20, 2002 Ab initio clcultions t the B3LYP nd MP2 levels suggest tht series of compounds with the generl formul N 5 MN 7 (M ) Ti, Zr, Hf, Th) re loclly stble. These compounds re thermodynmiclly t lest s stble s the recently suggested ScN 7 molecule. N 5 ThN 7 seems the most stble of ll. It lies 21.5 kcl/mol below trnsition stte, corresponding to the opening of one N-N bond in the N 7 ring, nd only 132 kcl/ mol bove Th + 6N 2, or 22 kcl/(mol N 2 ). Introduction The isoltion of stble slts of the N + 5 ction, the first new ll-nitrogen species to be mde in century 1,2 hs put the spotlight on the serch for other stble polynitrogen species. We hve recently predicted the possible existence of the ScN 7 molecule, 3 which contins new structurl group, the η 7 -N π-electron ring, isoelectronic with the experimentlly known η 7 -(C 7 H 7 ) 3- lignd, nd hs reltively low energy of formtion of 36 kcl/mol per N 2 unit, bove Sc tom nd N 2. The ide behind this study is tht 10π system like N 3-7 hs occupied σ, π, nd δ shells cpble of dontion to the M 3+ metl. The interction between the metl (M) nd the poly-nitrogen cluster should thus stbilize the compound with respect to N q n species. In this study, we consider the further complextion of such MN 7 moiety to six-π-electron, η 5 -N - 5 pentzole ring. This gives rise to sndwich compounds with the generl formul N 5 MN 7 (Figure 1). We investigted the compounds contining group 4 element, M ) Ti, Zr, Hf, plus Th. Along row 4, the species contining Sc, Ti, nd V, with totl chrge of -1, 0, nd +1, respectively, were lso investigted. It turns out tht the energies of formtion of this clss of molecules re comprble with the previous ScN 7 cse, even t the high N-to-M rtio of 12. Furthermore, the M tom in these new species is stericlly shielded, wheres tht of ScN 7 ws open (unless the ScN 7 compound would crystllize s chins). The trnsition-metl complexes of the η 5 -N - 5 lignd were briefly considered by us 3 nd thoroughly studied by Lein et l. 4 in the cse of the ferrocene nlogue Fe(N 5 ) 2. Its energy of formtion ws, however, found to be somewht higher, 40 kcl/ mol per N 2 unit bove the known Fe + 5N 2. Computtionl Detils The clcultions were crried out t the density functionl theory (DFT) level, using the B3LYP exchnge-correltion functionl, with 6-31G* bsis set for the nitrogen toms nd metl tom, M ) Sc, Ti, nd V. When M belonged to the fifth, * To whom correspondence should be ddressed. E-mil: lur.g@ cim.unibo.it. Università di Bologn. University of Helsinki. Figure 1. Structure of the locl minimum of N 5MN 7 (M ) Ti, Zr, Hf, nd Sc). M ) Zr, sixth, M ) Hf, nd seventh row, M ) Th, effective core potentils (ECPs) were used on the metl tom. The energydjusted Stuttgrt ECPs were used for this purpose. 5 The number of core electrons ws 28 for Zr nd 60 for Hf nd Th. The ccompnying bsis set of the ECPs (8d,7p,6d,4f) ws used to describe the vlence electron density. 6 The progrm Gussin 98 ws employed. Equilibrium geometries nd hrmonic frequencies were computed for ll species. The clcultions were performed using the ultrfine (99, 590) grid hving 99 rdil shells nd 590 ngulr points per shell. At the sme time, the weighting scheme of Becke hs been used for numericl integrtion. The grdient /jp CCC: $ Americn Chemicl Society Published on Web 03/27/2002

2 New η 5 -N 5 - -Metl-η 7 -N 7 3- Compounds J. Phys. Chem. A, Vol. 106, No. 18, TABLE 1: B3LYP Bond Lengths (Å) nd Angles (degrees) for the N 5 MN 7 Species, with M ) Ti, Zr, Hf, nd Sc system N 5TiN 7 N 5ZrN 7 N 5HfN 7 N 5ThN 7 N 5ScN 7 - R M-N (2.109) (2.228) R M-N (2.317) (2.492) R N7-N (1.392) (1.406) R N5-N (1.362) (1.360) N7MN (38.5) (36.8) N5MN (34.2) (31.7) MP2 Vlues in Prenthesis for N 5TiN 7 nd N 5HfN 7. N5 is nitrogen tom lying on the N 5 ring, nd N7 is nitrogen tom lying on the N 7 ring. tolernce ws the defult one in Gussin which is set to be 10-6 on the lrgest component of the Crtesin grdient. Comprtive geometry optimiztions nd hrmonic frequency clcultions were performed using MP2 for some of the species. Results In Tble 1, the typicl bond distnces nd ngles for the vrious molecules tht were found to be stble re reported. All of them hve C s symmetry (the symmetry plne of the molecules is plne perpendiculr to the N 5 nd N 7 rings, contining the metl tom nd one N tom on the N 5 nd one on the N 7 ring). Along group 4, N 5 TiN 7,N 5 ZrN 7, nd N 5 HfN 7, together with N 5 ThN 7, re locl minim. Along row 4, N 5 ScN 7 - nd N 5 TiN 7 re locl minim, wheres N 5 VN 7 + is not stble nd dissocites into two N 2 groups, one N 3 moiety, one N 5 moiety, nd V tom. We checked the stbility of the triplet for N 5 TiN 7, nd it ws found to lie 15 kcl/mol higher in energy thn the singlet. In ll stble structures, the N 7 ring is closer to the M tom thn the N 5 ring, with the typicl M-N7 distnce (N7 is nitrogen tom lying on the N 7 ring) being c. 0.3 Å shorter thn the typicl M-N5 distnce (N5 is nitrogen tom lying on the N 5 ring). In N 5 TiN 7, these M-N distnces re smller thn in ny other system. Along group 4, going from Ti to Zr, the two M-N distnces become c. 0.2 Å longer, wheres they become 0.02 Å shorter in going from Zr to Hf. In N 5 ThN 7,M-N7 is 0.2 Å longer nd M-N5 is 0.3 Å longer, thn in N 5 HfN 7. Along row 4, in going from Sc to Ti, M-N7 becomes 0.1 Å longer nd M-N5 becomes 0.2 Å longer. One TABLE 2: Hrmonic Frequencies (cm -1 ) nd Their IR Intensities (km mol -1 ) in Prenthesis for the N 5 MN 7 Species (M ) Ti nd Zr), t the B3LYP nd MP2 Levels of Theory norml mode N 5TiN 7/B3LYP N 5TiN 7/MP2 N 5ZrN 7/B3LYP ν 1() N 5,N 7 rottion i5.8 (0) (0) i9.5 (0) i8.7 (0) ν 2(e) M-N 5,M-N 7 bend 78.3 (2) (2) 77.5 (2) 35.9 (5) ν 3(e) M-N 5 bend (0) (1) (0) (0) ν 4() M-N 5,M-N 7 sym. stretch (7) (2) (2) (15) ν 5(e) N 7 out of plne brething (0) (3) (0) (0) ν 6(e) M-N 7 stretch (1) (1) (1) (0) ν 7(e) N 7 out of plne bend (0) (1) (0) (0) ν 8() M-N 5,M-N 7 sym. stretch (116) (17) (99) (104) ν 9(e) N 7 out of plne bend (0) (0) (0) (0) ν 10(e) N 5 out of plne bend (0) (0) (0) (0) ν 11() N 7 in plne brething (13) (43) (14) (9) ν 12(e) N 7 out of plne def (0) (0) (0) (0) ν 13(e) N 5,N 7 out of plne def (0) (0) (0) (0) ν 14(e) N 7 in plne brething (14) (2) (8) (21) ν 15(e) N 7 in plne def (0) (2) (0) (0) ν 16(e) N 7 in plne sym def (0) (0) (0) (0) ν 17(e) N 5 in plne sym stretch (0) (3) (0) (0) ν 18() N 5 in plne brething (0) (29) (0) (1) ν 19(e) N 5 in plne brething (3) (2) (2) (5) For N 5TiN 7/B3LYP, the Rmn intensities re lso reported s the second quntity in prentheses. TABLE 3: Hrmonic Frequencies (cm -1 ) nd Their IR Intensities (km mol -1 ) in Prenthesis for the N 5 MN 7 Species (M ) Hf, Th, nd Sc), t the B3LYP nd MP2 Levels of Theory norml mode N 5HfN 7/B3LYP N 5HfN 7/MP2 N 5ThN 7/B3LYP N 5ScN - 7 /B3LYP ν 1() 3.4 (0) (0) i9.7 (0) i8.5 (0) (0) i10.6 (0) ν 2(e) 41.1 (4) (1) 37.1 (4) 35.7 (11) (0) 62.9 (5) ν 3(e) (0) (1) (0) (0) (2) (0) ν 4() (17)(0) (10) (38)(0) (8) ν 5(e) (0) (3) (0) (1) (3) (3) ν 6(e) (0) (5) (0) (0) (3) (0) ν 7(e) (0) (0) (0) (0) (2) (0) ν 8() (66)(20) (69) (109) (13) (134) ν 9(e) (0) (0) (0) (0) (0) (0) ν 10(e) (0) (0) (0) (0) (0) (0) ν 11() (6) (45) (11) (10) (37) (14) ν 12(e) (0) (3) (0) (0) (3) (0) ν 13(e) (0) (1) (0) (0) (0) (0) ν 14(e) (24)(2) (16) (30) (1) (43) ν 15(e) (0) (0) (0) (0) (2) (0) ν 16(e) (0) (0) (0) (0) (0) (0) ν 17(e) (0) (3) (0) (0) (2) (0) ν 18() (0) (34) (0) (1) (24) (0) ν 19(e) (5) (1) (3) (4) (0) (9) For N 5HfN 7/B3LYP nd N 5ThN 7/B3LYP the Rmn intensities re lso reported s the second quntity in prentheses. The ssignment is the sme s in Tble 2.

3 4692 J. Phys. Chem. A, Vol. 106, No. 18, 2002 Gglirdi nd Pyykkö TABLE 4: Prtil Chrges on N 5, M, nd N 7 nd Totl Dipole Moment (Debye) system/method δ N5 δ M δ N7 dipole N 5TiN 7/B3LYP N 5TiN 7/MP N 5ZrN 7/B3LYP N 5HfN 7/B3LYP N 5HfN 7/MP N 5ThN 7/B3LYP N 5ScN 7-/B3LYP ScN 7/B3LYP b ScN 7/MP2 b The origin of the system is on the Sc tom. b Reference 3. should ber in mind tht the Sc compound hs totl chrge of (-1), wheres ll other structures re neutrl. The typicl N-N distnces in both the two rings re rther constnt when vrying the centrl metl. For the Ti nd Hf compounds, the MP2 structures were lso optimized. The MP2 bond distnces re similr to the B3LYP vlues. In Tbles 2 nd 3, the B3LYP hrmonic frequencies of the molecules re reported, together with their IR intensities. All species except N 5 HfN 7 hve one very smll imginry frequency corresponding to the rottion of the two rings one with respect to the other. N 5 HfN 7, on the other hnd, hs smll rel frequency corresponding to this purely rottionl mode. Locl minim should show zero imginry frequencies. However, in this cse, the smll imginry frequency corresponds to purely rottionl mode. We followed this mode in N 5 TiN 7 nd ended up into distorted structure which ws less thn 10 cm -1 lower in energy thn the C s one. We cn thus sfely sy tht the molecule might rotte, but sttisticlly, it will ssume most of the time the undistorted C s conformtion. Most of the modes hve zero IR intensities, except for few modes: ν 2, corresponding to M-N 7 nd M-N 5 bending; ν 4 corresponding to M-N 7 nd M-N 5 symmetric stretching; ν 8 corresponding to M-N 7 nd M-N 5 ntisymmetric stretching, which is the most intense mode in ll molecules; ν 11 corresponding to N 7 in plne brething; ν 14 corresponding to N 7 in plne brething; nd ν 19 corresponding to N 5 in plne brething. For N 5 TiN 7, N 5 HfN 7, nd N 5 ThN 7, the B3LYP Rmn intensities re lso reported. Most of the modes hve very little Rmn intensity. The only modes hving some Rmn intensity re ν 8, ν 11, nd ν 18. The MP2 frequencies hve lso been clculted for N 5 TiN 7 nd N 5 HfN 7, nd they re similr to the B3LYP frequencies. In N 5 HfN 7, ν 1 hs n imginry vlue of i9.7 cm -1. In Tble 4, the prtil chrges on the N 5,M,ndN 7 moieties, obtined by Mulliken popultion nlysis, re reported. Formlly, they correspond to N 5 -,M 4+ (except for Sc 3+ ), nd N The rel chrge is however smller thn the forml chrge. The totl dipole moments re lso reported. For the chrged species N 5 ScN 7 -, the origin of the system ws plced on the Sc tom. At the B3LYP level, the ionicity increses in going from Ti to Zr to Hf, together with the totl dipole moment. N 5 ThN 7 hs the lrgest dipole moment of ll, but the prtil chrges on the moieties re lower thn in ll other systems. MP2 gives lrger ionicity nd dipole moment thn B3LYP. A possible dissocition mechnism ws investigted for N 5 TiN 7, in nlogy with wht ws previously done for ScN 7. 3 We considered the opening of the N 7 ring nd found trnsition stte (TS; Figure 2), with one imginry frequency, which lies 9 kcl/mol higher in energy thn the locl minimum (LM) N 5 TiN 7. This energy brrier of 9 kcl/mol lredy includes zero-point energy correction of 1 kcl/mol. For ScN 7, the Figure 2. Structure of the trnsition stte of N 5MN 7 (M ) Ti nd Hf). nlogous energy brrier ws of 20 kcl/mol. Moreover, the TS for ScN 7 occurs when the N-N bond opens to 2.10 Å, wheres in the N 5 TiN 7 TS, the N-N bond is only 1.86 Å. This indictes tht presumbly the opening process is esier to strt in N 5 TiN 7 thn in ScN 7. As possible dissocition products of N 5 TiN 7, we considered six N 2 molecules nd Ti tom. These lie 210 kcl/mol lower in energy thn N 5 TiN 7. Per N 2 unit, the ltter corresponds to 35 kcl/mol. An nlogous energy blnce for ScN 7 gve 36 kcl/mol per N 2 unit. The nlysis of the structure nd frequencies of N 5 ZrN 7 did not suggest tht the Zr compound could be more stble thn the Ti compound. N 5 HfN 7, on the other hnd, hs ll rel frequencies t the B3LYP level of theory. Compred to N 5 ZrN 7, the two typicl M-N bond distnces re slightly shorter. The close chemicl similrity between Zr nd Hf cn be ttributed to cncelltion by shell-structure effects by reltivistic effects. 7,8 We re not wre of ny studies on the role of reltivistic effects on the bonding of group 4 metls to 6π or 10π romtic rings. The usul opening of the N 7 ring presents TS (Figure 2), with one imginry frequency, which lies 15 kcl/mol higher in energy thn the N 5 HfN 7 locl minimum, including zero point energy correction of 1 kcl/mol. In the TS the N7-N7 bond is 2.03 Å. With respect to 6 N 2 molecules nd Hf tom in its triplet ground stte, N 5 HfN 7 lies 180 kcl/mol higher in energy, which corresponds to only 30 kcl/mol per N 2 unit. In N 5 ThN 7 the ring opening presents TS with one imginry frequency which lies 21.5 kcl/mol higher in energy thn the corresponding locl minimum, lredy including zero point energy correction of 2 kcl/mol. The N7-N7 bond is 2.06 Å in the TS. With respect to 6 N 2 molecules nd Th tom in its triplet ground stte, N 5 ThN 7 lies 132 kcl/mol higher in energy, which corresponds to only 22 kcl/mol per N 2 unit. N 5 ThN 7 is thus the most stble of ll these N 5 MN 7 molecules. It lso seems tht the dissocition process is more difficult to strt in N 5 ThN 7 in ScN 7, with its energy brrier of 20 kcl/mol.

4 New η 5 -N 5 - -Metl-η 7 -N 7 3- Compounds J. Phys. Chem. A, Vol. 106, No. 18, Figure 3. Some Hrtree-Fock moleculr orbitls (MO) of N 5TiN 7: from MO 37 to MO 56. The orbitl energy (.u.) is lso reported. MO 53 is the HOMO, nd 54 is the LUMO. The totl electron density in the bottom left-hnd corner. We performed n orbitl nlysis on the vrious species. As n exmple, we consider in detil the bonding of N 5 TiN 7 using the Hrtree-Fock orbitls. Including ll of the 2s nd 2p electrons of the rings nd the four electrons of the metl, there re in ll 64 electrons, or 32 doubly occupied MOs to nlyze. These rnge from the HOMO, orbitl 53, down to MO 21. The shpes of MOs from 53 to 34 re shown in Figure 3, together with three virtul orbitls, the LUMO, orbitl 54, nd MOs 55 nd 56. A schemtic qulittive energy-level digrm is reported in Figure 4. Our first expecttion ws tht the bonding would minly be between the ring 2pπ orbitls nd the metl d orbitls. Around the moleculr symmetry xis there would be σ nd π orbitls binding to both the N 5 ring nd the N 7 ring nd, in ddition δ combintion, binding to the N 7 ring. Such orbitls cn indeed be identified. The two σ MOs re 34 nd 37, respectively. The π bond to N 5 is formed by MOs Wht complictes the sitution is tht the seven-ring N (2s+2p)σ orbitls lso hybridize with the metl. From the N 7 ring, this yields the four xilly π MOs, 38-41, nd the four xilly δ MOs, 50-53, ll hybridized with the metl dδ. In ddition, we hve MOs describing other intrring σ orbitls in this prt of the eigenvlue spectrum. MO 54 is the metl d 0 LUMO tht would receive n electron in n nion rdicl. The totl electron density is shown in the lower left-hnd corner of Figure 3. Given the incresing of stbility in going from lighter to hevier metls (form Ti to Th), we investigted the possibility tht lso the compound contining urnium (IV) could exist, but this turned out to be unstble. We lso studied the N 5 ScN 7 - nion. This system lies 69 kcl/ mol lower in energy thn ScN 7 nd N 5 -. In Figure 5, the totl energy of N 5 ScN 7 - s function of the distnce between ScN 7 nd N 5 - is reported. At ech vlue of the distnce between ScN 7 nd N 5 -, the two moieties were reoptimized. The energy of the system increses with the distnce up to 10 Å nd then reches the symptotic vlue. It thus seems there is no brrier to the formtion of the supersystem from the two frgments. Conclusions We presented the results of study on some complexes with generl formul N 5 MN 7, where M is group 4 tom, Ti, Zr,

5 4694 J. Phys. Chem. A, Vol. 106, No. 18, 2002 Gglirdi nd Pyykkö Figure 4. Qulittive energy-level digrm of N 5TiN 7 moleculr orbitls. the lower energy of N 5 ThN 7 with respect to the dissocition products compred to ScN 7, we suggest tht N 5 ThN 7 should hve chnce of existing. The bonding in these N 5 MN 7 species cn be understood using simple moleculr orbitl concepts nd noting tht in the N 7 ring, strong hybridiztion occurs between the ring σ nd π systems. The isoelectronic ionic species N 5 ScN 7 - turned lso out to be interesting, becuse it cn be formed vi brrierless rection from N 5 - nd ScN 7. Acknowledgment. This work ws prtilly supported by Ministero dell Università e dell Ricerc Scientific nd The Acdemy of Finlnd. We thnk Dr. Vler Veryzov, Lund University, for writing the interfce to Cerius2 tht ws used to generte the picture of the moleculr orbitls. Figure 5. Totl energy (.u.) of N 5ScN 7- s function of the distnce (Å) between ScN 7 nd N 5-. Hf, nd lso Th. These compounds re locl minim. The system contining Th is the most stble of ll. It presents brrier to dissocition of 21.5 kcl/mol, corresponding to the breking of N-N bond on the N 7 ring, nd it lies only 132 kcl/mol bove Th + 6N 2, or 22 kcl/(mol N 2 ). N 5 HfN 7 hs brrier of 15 kcl/mol nd lies 177 kcl/mol bove Hf + 6N 2, or 29.5 kcl/ (mol N 2 ). The previously predicted ScN 7 molecule hs brrier to dissocition of 20 kcl/mol nd lies 36 kcl/(mol N 2 ) bove Sc nd 7/2 N 2. The opening process presumbly strts esier in ScN 7 thn in the Th sndwich compound. Moreover, considering References nd Notes (1) Christe, K. O.; Wilson, W. W.; Sheehy, J. A.; Botz, J. A. Angew. Chem. Int. Ed. Engl. 1999, 38, (2) Vij, A.; Wilson, W. W.; Vij, V.; Thm, F. S.; Sheehy, J. A.; Christe, K. O. J. Am. Chem. Soc. 2001, 123, (3) Gglirdi, L.; Pyykkö, P. J. Am. Chem. Soc. 2001, 123, (4) Lein, M.; Frunzke, J.; Timoshkin, A.; Frenking, G. Chem. Eur. J. 2001, 7, (5) Andre, D.; Heussermnn, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Act 1990, 77, 123. (6) Institut für Theoretische Chemie, Universität Stuttgrt. ECPs nd corresponding Vlence bsis sets. (7) Pyykkö, P.; Desclux, J. P. Chem. Phys. Lett. 1977, 50, 503. (8) Pyykkö, P.; Snijders, J. G.; Berends, E. J. Chem. Phys. Lett. 1981, 83, 432.